The present invention generally relates to an insulation barrier on a printed circuit board, and more particularly to a printed circuit board assembly provided with an insulation transformer, and manufacturing of such a printed circuit board assembly.
In order to insulate electrical systems or circuits from each other, insulation transformers are normally employed. An insulation transformer is designed to provide magnetic coupling between electrical systems without introducing significant conductive or electrostatic coupling between them. Insulation is mainly a question of separating connections and components, and hence it is evident that the insulation requirements work against minimizing size of the overall equipment. Accordingly, the construction of insulation transformers typically requires significant clearance and creepage distances to obtain the desired electrical insulation. Here, creepage distance means the shortest distance between two conductors measured along the surface of the insulation.
FIG. 1 is a schematic diagram illustrating an insulation transformer provided on a printed circuit board (PCB) according to the prior art. The insulation transformer 1 is generally mounted on the PCB 2 by means of conventional transformer pins, a primary side transformer pin 3 and a secondary side transformer pin 4. An insulation transformer for use on a printed circuit board (PCB) is generally designed such that the point-to-point distance along a straight line between the primary side and the secondary side of the transformer is dimensioned for the creepage distance of the PCB. This means that the point-to-point distance between the primary side and the secondary side of the transformer has to be larger than the minimum required creepage distance of the PCB at the relevant voltages.
The comparative tracking index (CTI) is a comparative index that can be used to compare insulating materials with respect to creepage distance. The higher the CTI value, the smaller the required creepage distance. This means that for a small creepage distance, an insulating material of a high CTI value has to be selected.
Although printed circuit boards can be made of a wide variety of materials, they are indeed often made of materials, such as FR4 or similar, which have relatively low CTI values. It is possible to produce PCBs from materials that have higher CTI values, but this may require the use of specialized and expensive materials for the PCB. In practice, the use of such specialized materials is rare, and printed circuit boards generally have relatively low CTI values. Consequently, the required creepage distance becomes rather long. This means that the size of the transformer will have to be relatively big, and that a rather large area of the PCB will be occupied by the transformer.
The present invention overcomes these and other drawbacks of the prior art arrangements.
U.S. Pat. No. 5,726,616 issued to Bell on Mar. 10, 1998 relates to a transformer with plural bobbins. An assembly cap is fitted around the bobbins and includes flanges for locating a core. The bobbins and the cap are designed such that creepage and clearance requirements are met. The core comprises laminations, which are fitted through the assembly cap and the bobbins, and the cap provides a receptacle for the core. To secure the laminations in position, an assembly cup is mounted around the outside of the laminations.
It is a general object of the present invention to provide a volume-efficient insulation barrier on a printed circuit board (PCB) or PCB assembly.
In particular it is desirable to provide the required insulation between two electrical systems on a PCB or PCB assembly by means of a relatively small and compact insulation transformer mounted thereon.
Another object of the invention is to improve the creepage characteristics of an insulation transformer used as an insulation barrier on a PCB or PCB assembly.
Still another object of the invention is to provide an efficient method for manufacturing a printed circuit board assembly comprising an insulation transformer.
These and other objects are met by the invention as defined by the accompanying patent claims.
The general idea according to the present invention is to use the insulation transformer as a support structure to mechanically interconnect at least two separate PCB sections. Preferably, the PCB sections are formed by dividing a PCB into sections after mounting of the insulation transformer to the PCB. Since the PCB has been separated, it no longer affects the creepage distance for the insulation barrier. Instead the insulating material of the transformer determines the minimum required creepage distance. The insulating material of the transformer generally has a considerably higher CTI value than the PCB material, leading to a relatively short creepage distance being required. By breaking the creepage path on the PCB, it is thus possible to use a more balanced design of the whole printed circuit board assembly. The following advantages are obtained:
For any given insulation requirement, the size of transformer will be substantially reduced compared to the prior art arrangements.
Savings of material for transformer housing and resin.
The penalty area on the PCB for the transformer will be reduced.
The lower transformer weight makes it easier to handle printed circuit board vibrations.
Since the PCB is divided into separate sections, it is no longer necessary to test the PCB at manufacturing to confirm the required insulation, thus eliminating a cost-intensive and time-consuming test procedure.
It should be understood that although an efficient manufacturing of such a PCB assembly normally implies that a PCB is divided into separate sections after mounting of the insulation transformer to the PCB, it is possible to provide the PCB sections as separate pieces from the very beginning.
In order to improve the creepage characteristics of the insulation transformer, the transformer housing is preferably formed in a substantially non-planar manner. Advantageously, the transformer housing is formed with recessed sections, such as slots or similar, and/or projecting sections, such as fins or ribs. In this manner, the distance between the primary side and the secondary side of the transformer along the surface of the transformer housing is increased. In this way, the insulation transformer can be made even more compact.
An efficient way of manufacturing a printed circuit board assembly according to the invention involves making an opening and snap-off sections in the PCB. The snap-off sections are formed on each side of the opening such that the PCB is divided into separate sections when the snap-off sections are removed after mounting of the insulation transformer. First, however, the insulation transformer is placed aligned with the opening and mounted to the PCB such that the PCB will act as a support structure that mechanically interconnects the separate PCB sections. Preferably, the snap-off sections are removed after mounting of the transformer and other components has been completed. In this way, the snap-off sections may provide the required stiffness to the PCB for optimal soldering.
By using a so-called snap-off board technique, it is still possible to efficiently manufacture the printed circuit board assembly on an automated soldering line.
Other advantages offered by the present invention will be appreciated upon reading of the below description of the embodiments of the invention.